1. Field of the Invention
The present invention relates to a control apparatus that calculates a control input based on a value calculated by a feedback control method and a value calculated by a feedforward control method, to thereby control a controlled variable using the control input, a control method, and an engine control unit.
2. Description of the Related Art
Conventionally, as a control apparatus of this kind, the present assignee has already proposed a control apparatus disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2005-315161. This control apparatus controls the air-fuel ratio of a mixture in an internal combustion engine as a controlled variable, based on a fuel amount as a control input, and is comprised of an air flow sensor that detects the flow rate of air flowing through an intake passage of the engine, a pivot angle sensor that detects a valve lift, a cam angle sensor that detects the phase of a camshaft for actuating an intake valve to open and close the same, relative to a crankshaft (hereinafter referred to as “the cam phase”), and a crank angle sensor. Further, the engine includes the intake passage having a large diameter, as well as a variable valve lift mechanism and a variable cam phase mechanism as variable intake mechanisms. In the engine, the valve lift and the cam phase are changed as desired by the variable valve lift mechanism and the variable cam phase mechanism, respectively, whereby the amount of intake air is changed as desired.
In the above control apparatus, as an intake air amount, a first estimated intake air amount is calculated in a low-load region based on the valve lift and the cam phase, and in a high-load region, a second estimated intake air amount is calculated based on the flow rate of air. In a load region between the low-load region and the high-load region, a weighted average value of the first and second estimated intake air amounts is calculated. This is because in the low-load region where the reliability of the second estimated intake air amount is lower than that of the first estimated intake air amount due to the large diameter of the intake system of the engine, the first estimated intake air amount higher in reliability is employed, whereas in the high-load region in which occurs a state opposite to the above state in the low-load region, the second estimated intake air amount higher in reliability is employed. Further, a basic fuel amount is calculated as a value for use in feedforward control of the air-fuel ratio based on the thus calculated intake air amount, and an air-fuel ratio correction coefficient is calculated with a predetermined feedback control algorithm such that the air-fuel ratio is caused to converged to a target air-fuel ratio. A final fuel amount is calculated based on a value obtained by multiplying a basic fuel amount by the air-fuel ratio correction coefficient. Then, this amount of fuel is injected into cylinders via fuel injection valves, whereby the air-fuel ratio is accurately controlled such that it becomes equal to the target air-fuel ratio.
According to the above-described control apparatus, when detection signals from the pivot angle sensor, the cam angle sensor, and the crank angle sensor drift due to changes in temperature, for example, or when the static characteristics of a variable valve lift mechanism and a variable cam phase mechanism (i.e. the relationship between the valve lift and the cam phase with respect to the control input) are changed by wear of components of the two variable mechanisms, attachment of stain, and play produced by aging, the reliability of the results of detection by the sensors lowers, which can result in a temporary increase in the control error of the air-fuel ratio. More specifically, when the first estimated intake air amount ceases to represent an actual intake air amount, and deviates from the actual intake air amount, there is a fear that the fuel amount cannot be properly calculated as a control input in the low load region where the first estimated intake air amount is used as the control input. In such a case, the difference between the air-fuel ratio as the controlled variable and the target air-fuel ratio, that is, the control error increases. Although the control error can be compensated for by the air-fuel ratio correction coefficient in a steady state since the air-fuel ratio correction coefficient is calculated with the predetermined feedback control algorithm, it takes time before the control error is compensated for by the air-fuel ratio correction coefficient. Therefore, e.g. when the control error temporarily increases, the accuracy of control is temporarily degraded, which results in unstable combustion and degraded combustion efficiency. The problem described above is liable to be more conspicuous in a transient operating state of the engine.